1. Field of the Invention
The present invention relates to a method of processing marine seismic data, and in particular relates to a method of processing marine seismic data that includes two modes of seismic energy propagation, one mode arising from partial mode conversion at a boundary of a layer of the earth. The method of the invention provides an estimate of static shifts caused by a layer at or near the earth's surface that has different seismic properties from the underlying layers. The invention also relates to a method of seismic surveying that includes processing acquired seismic data in the above way.
2. Description of the Related Art
FIG. 1 is a schematic view of a marine, seabed seismic surveying arrangement. In this surveying arrangement, seismic energy is emitted by a seismic source 1 that is suspended beneath the sea-surface from a towing vessel 2. The seismic energy is emitted downwards, passes into the earth's interior and is reflected by a geological feature that acts as a reflector 3. The reflected seismic energy passes upwards through the earth's interior, into the sea and is detected by a seismic receiver 4 disposed on the sea-bed. Information about the earth's interior is obtained, for instance, by determining the travel time of seismic energy from the seismic source 1 to the receiver 4. Only one reflector 3 is shown in FIG. 1 but, in practice, a number of geological features within the earth's interior will act as partial reflectors for the seismic energy. Moreover, only one source 1 and one receiver 4 are shown in FIG. 1 but in practice a marine seismic surveying arrangement may have an array of sources and an array of receivers.
The geological structure of the earth is not uniform. One problem in processing marine seismic data is that frequently there is a layer 5 at or near the surface whose properties may well be significantly different from the properties of the underlying geological structure 6 hereinafter referred to as the “basement”). This can occur if, for example, there is a layer at or near the earth's surface that is less consolidated than the basement. In particular, the velocity of seismic energy may be significantly lower in the surface or near-surface layer 5 than in the basement 6, and such a surface or near-surface layer is thus generally known as a “low-velocity layer” (or LVL). This difference in velocity will produce a shift in the travel time of seismic energy compared to the travel time that would be recorded if the surface or near-surface layer and the basement had identical seismic properties, and these shifts in travel time are generally known as “static shifts”, or just “statics”.
The low-velocity layer 5 is shown as a surface layer FIG. 1, but it need not extend to the surface and there could be a further layer overlying the low-velocity layer.
The static shift generated by a surface or near-surface low-velocity layer 5 depends on the thickness of the layer, and on the velocity of propagation of seismic energy through the layer. Lateral variations usually occur in both the thickness of a low-velocity layer 5 and the propagation velocity through the layer, so that the static shift observed at a seismic receiver at one location is likely to be different from the static shift observed at a receiver at another location. To a first approximation, the entire data set recorded at one receiver will be advanced or delayed by a static time shift relative to data recorded at another receiver.